Storage systems



y 1966 J. J. MITCHELL 3,260,062

STORAGE SYSTEMS Filed Oct. 2, 1964 ATTORNEYS United States Patent3,260,062 STORAGE SYSTEMS James J. Mitchell, Houston, Tex., assignor toTexas Eastern Transmission Corporation, Shreveport, La., a corporationof Delaware Filed Oct. 2, 1964, Ser. No. 401,158 4 Claims. (Cl. 6254)This invention relates to storage systems, and more particularly toliquid storage systems having an substantial boilotf in which the systemis protected against excess differential pressure being exerted acrossthe storage tank.

It is an object of this invention to protect a large tank from internalor external changes in pressure.

Another object is to provide a large liquid storage tank which utilizesa compressor for withdrawing boi-loff from the liquids with a system forcontrolling the pressure with in the tank relative to the outsidepressure Without shutting down the compressor.

Another object is to provide a system as in the preceding object withmeans for adding vapor or venting vapor from the tank if the controlsystem is incapable of quickly reducing a pressure differential acrossthe tank wall.

Another object is to provide a storage system as in the preceding twoobjects with a final two-way safety valve which, after all othercontrols have proved inadequate, will open the tank to atmosphere and,thus, reduce the pressure differential between the interior and exteriorof the tank.

. Other objects, features and advantages of the invention will beapparent from the drawing, the specification and the claims.

The drawing is a single figure, partly in cross-section and partly inelevation, illustrating the preferred form of this invention.

This invention is particularly adapted for use in connection with thestorage of liquefied natural gas, and the description will be related tothe storage of liquefied natural gas. It will be understood that otherliquids may be stored in a tank and the invention find usefulness inprotecting the tank against a diiferential in the same manner as whereliquefied natural gas is stored.

The tank indicated generally at is preferably positioned with a portionof the tank below ground level. However, it will be understood that thetank may be entirely above ground level. The tank includes a layer ofinternal insulation material 11 to control transmission of heat. It willbe noted that the insulation material 11a forms a false roof in thetank. The insulation material is such that there is free communicationbetween the tank above and below the false roof 11a. For all practicalpurposes, the area above insulation 11a and below the dome 10a of thetank is a dead vapor space.

Liquefied natural gas is introduced into the tank through the conduit 12and is withdrawn from the tank by the pump indicated generally at 13which delivers the liquefied natural gas to the line .14.

i In accordance with this invention, a system is provided formaintaining the pressure in the tank Within a selected range of thebarometric pressure outside of the tank. This system includes the vaporwithdrawal line 15 for removing vapors from the tank, and the vaporreturn line 16 for returning vapors to the tank. Both of these lines areconnected to the tank with the end of the line being above the normalmaximum liquid level so that they will be circulating vapors only. Itwill be appreciated, however, that the vapor return line could extendbelow the liquid level as it could discharge vapors into the liquidwithoutaifectin-g the system. The vapor return line 16 is connected tothe vapor-withdrawal line 15 at point '17, so that these two lines maybe in communication.

Means for transferring vapors, which may be one or more compressors 20,are provided in the vapor-withdrawal line 15 between the tank and point17 and, in accordanoe with this invention, are run continuously.Normally, two compressors would be provided, with one being on standby.

A first valve means 18 is provided in the vapor-withdrawal line 15downstream of the connection with the vapor-return line 16, that is, atpoint 17. This valve 'controls the amount of vapors which aretransferred by the compressor 20 to the liquefaction plant forreliquefying and returning to the tank through the line 12.

A second valve means 19 is provided in the vapor-return line 16, andcontrols the amount of vapors which is returned to the tank by thecompressor 20. It will be apparent that the relative settings of the twovalves 18' and 19 may thus be used to return all of the withdrawn vaporsto the tank, transmit all of the withdrawn vapors to liquefaction, orany intermediate division of vapors so that a part is returned and apart is sent to liquefaction.'

Means are provided for controlling the setting of the valves 18 and 19to maintain the pressure in the vapor pressure readings and attempts tomaintain the pressure within the tank at a selected value relative tothe barometric pressure outside of the tank.

In accordance with the differences in pressures inside and outside thetank, the controller 21 signals the motors 18a and 19a to reposition thevalves 18 and 19 to vary the amount of vapor being returned to the tank.

In the preferred form, the controller 21 utilizes instrument air viaconduit 21a to operate the motors 18a and 19a and, as the pressuredifferential across the tank top 10a changes, the instrument airpressure to the motors 18a and 1911 will be changed. For instance, theinstruv ment air could increase in pressure with an increase in externalpressure relative to internal pressure. With such increase in pressure,it is desired to return more of the withdrawn vapors to the tank, andthe motor 18a will be of the type which will move the valve 18 moretoward closed position with an increase in instrument air pressure. Themotor 19a will be opposite in construction, so that with an increase ininstrument air pressure it will tend to open the valve 19. Of course, asthe instrument air pressure decreases due to pressure in the tankincreasing relative to external pressure, the opposite effect would beobtained, so that with a reduction in external pressure relative tointernal pressure the valve 18 would be moved to more open position andthe valve 19 to more closed position to reduce the internal tankpressure. 7 With the above explained construction, the compressor 20 maybe run continuously and the controller 21 will vary the setting ofvalves 18 and 19 as necessary to maintain the pressure within the vaporspace in the tank at a selective value relative to outside barometricpressure. This system will take care of normal boiloff and changes inboilofi? due to changes in ambient temperature, as well as most pressurechanges which are required due to changes in barometric pressure outsideof the tank.

In the event that the control system thus far explained 1 is incapableof handling a sudden differential of pressure brought on by anycondition such as a sudden large change in barometric pressure, meansare provided for Supplementing the pressure control provided by the twovalves 18 and 19. In the event the pressure within the tank is too highdue to a sudden fall in barometric pressure, a vent system is providedfor flaring some of the vapors to reduce the pressure within the tank.This system includes the vent line 24 which is controlled by the valve25. A suitable controller 26 senses the pressure within the tankimmediately below the dome through line 28. When the pressure within thetank relative to the barometric pressure outside of the tank is greaterthan a selected value, the valve 25 is opened by controller 26 and thevapor is flared through the flare tower indicated generally at 27.

In the event the pressure inside of the tank drops to a value too lowfor the system controlled by valves 18 and 19 to handle, then means areprovided for introducing auxiliary gas into the tank to bring thepressure up. This auxiliary gas will be of suitable composition to thevapors within the tank so that no contamination will result. Forinstance, the tank will be located adjacent a pipeline and auxiliary gasmay be obtained from a pipeline. In practice, the auxiliary gas will beavailable at many different sources within the liquefaction facilitiesand is piped to the tank through line 28.

The line 28 is controlled by the valve 29, which in turn is governed bythe action of controller 21. Instrument air pressure through supply line21a is fed to the selector relay 30 which controls the motor 29aassociated with valve 29. The selector relay is so arranged that thepressure at which it operates motor 29a to open valve 29 is one at whichthe valve 18 has been closed by the motor 18a, and all of the vaporsbeing withdrawn from the tank are recirculated to the tank. On the otherhand, the selector relay 30 will insure that the valve 29 is closed, atany time that valve 18 is opened, so that the warm pad gas will not beintroduced into the tank during any time that recirculating vapors areavailable for maintaining the pressure in the tank.

In the event that the auxiliary pressure control provided by the flaresystem and by auxiliary gas is insuflicient to take care of a suddendangerous pressure differential between interior and exterior of thetank in either direction, there is provided a two-way valve 31 on top ofthe tank which will open in either direction in response to a selectedrelatively high diflerential across the tank top. This type of controlis only utilized when all of the systems provided are incapable ofimmediately releasing the pressure differential because when externalpressure is high, operation of this valve permits contamination of thevapors Within the tank with outside air and may provide an explosivemixture. Additional explanation of operation of valve 31 due to high andlow barometric pressure is detailed in subsequent paragraphs.

In explaining the operation of the device, We may assume that a frontmoves over and causes a sudden drop in pressure. Up until this time, wemay assume that the system has been running on equilibrium with aportion of the vapor being returned to the tank and a portion of thevapor going to liquefaction facilities to be reliquefied and returned tothe tank. When the sudden drop in pressure occurs, the sensing element22 will record this drop,

. and the controller 21 will move the valve 18 toward open position andthe valve .19 toward closed position, so that vapors will be withdrawnfrom the tank to reduce its pressure. If the system is able to cope withthe drop in pressure and bring the tank down to the proper pressure, itwill do so and, when the pressures have been brought to the desiredstate, the valves 18 and 19 will be returned to an equilibrium position.

In the event that the closing of the valve 19 and opening of valve 18 isnot suflicient to bring the pressure within the tank down rapidlyenough, then the controller 26 will open valve 25 and some of the vaporsWill be flared to atmosphere. Of course, if the drop in barometricpressure is so rapid and so drastic that the tank is placed in dangerand is not relieved by the flare system, then the valve 31 will open anda large opening for release of vapors Will be provided to protect thetank. As soon as the excess vapors are released, the valve 31 will closeand thereafter the valve 25 of the flare system will be closed and thesystem returned to control by movement of valves 18 and 19.

In the event of a large increase in barometric pressure, the controller21 would close valve 18 and open valve 19 all the way. Then, if thenormal boilofl? of the liquid within the tank is sufficient to providethe necessary increase in internal pressure, the remainder of thecontrol system would remain inactive. In the event normal boilotf is notsutficient, then the selector relay 30 would open valve 29 and auxiliarygas would be introduced to raise the pressure within the tank. In theevent that these means for raising pressure are insuflicient, then as afinal stage the valve 31 would open and permit air to flow into the tankto equalize pressure.

The foregoing disclosure and description of the invention isillustrative and explanatory thereof and various changes in the size,shape and materials, as well as in the details of the illustratedconstruction, may be made within the scope of the appended claimswithout departing from the spirit of the invention.

What is claimed is:

1. A storage system comprising,

a storage tank,

a vapor withdrawal line connected to said tank above the maximum liquidlevel,

a compressor in said line,

a vapor return line connected to said tank and to said vapor withdrawalline downstream of said compressor,

first valve means in said vapor withdrawal line downstream of theconnection with said vapor return line,

second valve means in said vapor return line,

and pressure control means including a motor for each valve means and acontroller sensing pressure outside said tank and in the vapor spaceabove the maximum liquid level in said tank for moving said first valvemeans setting toward more closed position and said second valve meanssetting toward more open position with a reduction in pressure in thevapor space in said tank relative to the pressure outside said tank andvice versa.

2. A storage system comprising,

a storage tank,

a vapor withdrawal line connected to said tank above the maximum liquidlevel,

a compressor in said line,

a vapor return line connected to said tank and to said vapor Withdrawalline downstream of said compressor,

first valve means in said vapor withdrawal line downstream of theconnection with said vapor return line,

second valve means in said vapor return line,

a source of gas,

a supply line connecting said source and said tank,

third valve means controlling said supply line,

and pressure control means including a motor for each valve means and acontroller sensing pressure outside said tank and in the vapor spaceabove the maximum liquid level in said tank for moving said first valvemeans setting toward more closed position and said second valve meanssetting toward more open position with a reduction in pressure in thevapor space in said tank relative to the pressure outside said tank andvice versa,

said pressure control means normally maintaining said third valve meansclosed and opening said third valve means and completely closing saidfirst valve means when pressure within said tank vapor space drops to aselected value less than pressure outside exceeds a selected amountwhich is greater than can be said tank. equalized by said pressurecontrol means and said vent 3. The system of claim '2 wherein valvecontrol vent means. means is provided for said tank, andpressure-responsive References Cited by the Examiner means normallymaintains said vent means valve closed 5 UNITED STATES PATENTS and openssaid vent means valve when the pressure within the vapor space in saidtank exceeds the pressure out- 3150495 9/1964 Reed 62 54 side said tankby a selected amount. 3191395 6/1965 Mahel' et 62 54 4. The system ofclaim 3 wherein a normally closed 3210953 10/1965 Reed emergency two-wayvalve is provided in the tank and opens when the pressure differentialacross the tank top 10 ROBERT OLEARY Prima'y Examiner

1. A STORAGE SYSTEM COMPRISING, A STORAGE TANK, A VAPOR WITHDRAWAL LINECONNECTED TO SAID TANK ABOVE THE MAXIMUM LIQUID LEVEL, A COMPRESSOR INSAID LINE, A VAPOR RETURN LINE CONNECTED TO SAID TANK AND TO SAID VAPORWITHDRAWAL LINE DOWNSTREAM OF SAID COMPRESSOR, FIRST VALVE MEANS IN SAIDVAPOR WITHDRAWAL LINE DOWNSTREAM OF THE CONNECTION WITH SAID VAPORRETURN LINE, SECOND VALVE MEANS IN SAID VAPOR RETURN LINE, AND PRESSURECONTROL MEANS INCLUDING A MOTOR FOR EACH VALVE MEANS AND A CONTROLLERSENSING PRESSURE OUTSIDE SAID TANK AND IN THE VAPOR SPACE ABOVE THEMAXIMUM LIQUID LEVEL IN SAID TANK FOR MOVING SAID FIRST VALVE MEANSSETTING TOWARD MORE CLOSED POSITION AND SAID SECOND VALVE MEANS SETTINGTOWARD MORE OPEN POSITION WITHA REDUCTION IN PRESSURE IN THE VAPOR SPACEIN SAID TANK RELATIVE TO THE PRESSURE OUTSIDE SAID TANK AND VICE VERSA.